Characterization of PMK1 and MPG1, two proteins involved in appressorium formation by the rice blast fungus Magnaporthe grisea

Fernando Alberto Tenjo-Fernandez, Purdue University

Abstract

To penetrate the rice plant, the fungus Magnaporthe grisea develops an infectious structure called an appressorium that requires the recognition of a hydrophobic surface by the fungal spore. Two genes were cloned by our lab that are involved in appressorium development. The MPG1 gene encodes a hydrophobin, a new class of hydrophobic proteins that are implicated in various aspects of fungal morphogenesis. A model for the function of MPG1 during appressorium formation proposes that the protein may act as a structural component of the appressorium, and/or as an adhesion protein. To test this model, the localization of MPG1 was determined during appressorium formation, using an epitope-tagged MPG1 gene. The protein was detected on the spore surface and could not be detected on the appressorium. The MPG1 protein was detected at the tip of conidiophores and on the surface of immature spores. Thus, the results of these experiments show that MPG1 is a component of the spore surface, forming part of the rodlet layer present on the spore. This structure may contribute to the discrimination of hydrophobic surfaces to start appressorium formation. The PMK1 gene encodes a mitogen activated protein kinase (MAPK), similar to the FUS3 and KSS1 yeast MAPKs. PMK1 is essential for appressorium formation and invasive growth. The activation of PMK1 may affect its localization within the spore or the appressorium, or can be restricted to a particular cell type. The protein was detected in the spore and in the appressorium using a PMK1-GFP fusion protein. At late stages of appressorium development the protein seemed to be concentrated in the nucleus. A non-phosphorylatable allele and a catalytic inactive allele were generated to characterize the mechanism of PMK1 function during appressorium formation. In yeast, KSS1 represses or activates filamentous growth. The negative role only requires KSS1 presence, whereas its phosphorylation relieves the repression. Neither of the two PMK1 alleles complemented the appressorium defect of a pmk1 − strain, suggesting that both PMK1 phosphorylation and kinase activity are required for its function. This result shows that despite the homology between PMK1 and KSS1, the mechanism of action of MAPKs differs between fungal species.

Degree

Ph.D.

Advisors

Hamer, Purdue University.

Subject Area

Molecular biology|Microbiology

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